Loops in quantum gravity and dark energy – Updated

Loop corrections to quantum gravity may be able to explain dark energy without …

This article has been updated to correct my confusion of loop corrections to quantum gravity with loop quantum gravity.

Dark energy is the mystery du jour in cosmology. It is the name given to whatever is driving the current epoch of inflation, which is the accelerating rate of expansion of the universe. Unfortunately for theorists, we have exactly one data set that tells us something about dark energy—the rate of expansion of the universe—which is not really enough to test competing models. The knowledge that the expansion rate is increasing, however, is changing with many proposed observation programs aiming to obtain more data with increased accuracy in the next 10 years or so.

Despite popular opinion, theorists aren't willing to sit back and sharpen their pencils waiting for new data. No, they go right ahead and generate lots and lots of models, all of which fit the current data set (as they must) but which make differing predictions about the fine details. The simplest of these models features the return of the cosmological constant, appended to general relativity by Einstein to satisfy the then-prevalent view that the universe was static. The theory doesn't predict a value, instead relying on a highly accurate and suspiciously small value been input by the user of the theory. Of course, the situation may well be more complicated than that: what if the universe's rate of acceleration is changing at different rates at different places and times? No problem, replace the cosmological constant with a function that contains a number of rather accurate and suspiciously valued parameters obtained from measurement. In other words, none of these solutions tell us much about why the universe began to accelerate its rate of expansion at a particular time in the distant past or why that acceleration has continued to the present day.

This is where loop corrections to quantum gravity may well hold a unique position because they may actually make predictions regarding this most recent period of inflation. Now, if you don't understand what that means, you are not alone. I also have no clue what a loop correction to quantum gravity is. The best I have been able to discover is that when the universe was very young, the interactions between classical fields (e.g., electromagnetism) and virtual particles were also very strong. Apparently, one consequence of this is that virtual particles can "get trapped in the Hubble flow," which I take to mean that they experience a huge time dilation due to traveling at very close to the speed of light. This means that some of those particles only started to decay very recently (from our point of view) and may well have started the current epoch of inflation. In this case, the influence of the transition from a light-dominated universe to a matter-dominated universe, which occurred a very long time ago, shows up as the more recent decay of virtual particles that drive the recent new period of acceleration that we have labeled dark energy. The importance of this is that it gives a physical change in the universe to associate with something that we measure in the present day.

Now, I should note that this is not as cut and dried as it sounds. The researchers calculated some parameters from loop corrections to quantum gravity and used those to modify general relativity. From there, they show that the correct value for the onset of inflation and the rate of the rate of expansion are well predicted using only the approximate time of the light to matter transition. The major point is that the key parameters don't need to be that precise to generate a fit to the data and the parameters derived from quantum loop gravity are thought to arise naturally from the calculation—with the caveat that the equation cannot be solved so approximations were used for the current work. The result is that there is no need for fine tuning any parameters to get the universe we observe and that is far more satisfactory than arming yourself with an anthropic principle.

Oh no!

Chris Lee / Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.